Promotion of pyrene degradation in rhizosphere of alfalfa ( Medicago sativa L.)

Pot experiment was conducted to evaluate the phytoremediation of pyrene-contaminated soil using alfalfa ( Medicago sativa L.). Alfalfa biomasses, microbial viable counts, dehydrogenase activity, residual pyrene concentration and pyrene removal percentage were determined after 60 days of alfalfa growth. The results indicated that pyrene had an inhibitive effect on alfalfa growth, and higher pyrene concentration seriously affected alfalfa growth. In addition, the inhibitive effect on the root was more severe than that on the shoot. When pyrene concentration reached 492 mg kg −1 in soil, the shoot and root biomasses were only 34% and 22% of those of alfalfa growing in non-spiked soil, respectively. The rhizospheric bacterial and fungi counts were 5.0–7.5 and 1.8–2.3 times higher than those in non-rhizosphere soil, respectively. The residual concentrations of pyrene in the rhizosphere soil were lower than those in the non-rhizosphere soil. After 60 days, 69–85% and 59–80% of spiked pyrene disappeared from the rhizosphere and non-rhizosphere soils, respectively. The removal percentage decreased with increasing pyrene concentration. However, the average removal of pyrene in the rhizosphere soil was 6% higher than that in the non-rhizosphere soil. Therefore, the presence of alfalfa roots was effective in promoting the phytoremediation of freshly added pyrene into the soil.